There has been known a thread forming tap (cold forming tap) including a complete thread portion and a chamfered thread portion successive to the complete thread portion and having a radial size decreasing toward a leading end of the chamfered thread portion. An external thread alternately formed with protruding portions and radial relief portions is provided with the complete thread portion and the chamfered thread portion. Screwing the tap into a prepared hole of a workpiece at the chamfered thread portion allows the protruding portions to cut into a surface layer portion of an internal wall of the prepared hole. Then resultant plastic deformation at the surface layer portion of the internal wall of the prepared hole such that the internal thread is formed. Tools, disclosed in Patent Publications 1 and 2, represent such examples which are simplified in cleaning work owing to no disposal of chips.    Patent Publication 1: Japanese Patent Application Publication No. 1-289615    Patent Publication 2: Japanese Patent Application Publication No. 2005-205557